How did Cloud learn how to fight?

What makes you think he's actually good at fighting?

A team of 3 renegades can't take out 2 grunt type Soldier guards easily at the beginning of the game (Shinra HQ etc).

He's got pure strength on his side, presumably from Jenova, his training (as a grunt), and his mako infusion. With enough strength to hold that sword, you realize he's pretty strong and so if he swung it at you, you wouldn't really stand a chance.

Unless you deliberately power level him, he's never really a better fighter than the others in his party, and you realise Tifa (only) seems to know some form of kickboxing so you'd expect his huge sword to be far more dangerous than her?

Cloud did start out as only a grunt which gave him his "basic" fighting abilities and in real that is all you need to get better is the basics, but after he got his lucky shot in on Sephiroth because Sephiroth had his back turned and wasn't paying attention, Sephiroth had assumed there was no one left that could pose a threat to him. Cloud only managed to stick Zack's sword through Sephiroth out of pure luck, having not killed Sephiroth and gone after him when Sephiroth went to walk off, he ended up with Sephiroth's sword stuck through him. Even Sephiroth thought that should have killed him a weapon that kills all other things, instead Cloud took hold of the sword and threw him off the side down to the bottom of the reactor. That's the part of the story you may know from the game, what they don't show you in the game though is what happened to both Zack and Cloud after that... Shinra showed up with Professor Hojo who made both Zack and Cloud his new Experiment, Injected Jenova Cells into him in an attempt to turn Cloud into another Sephiroth, however Zack woke up earlier than scheduled due to seeing Angeal and broke both himself and Cloud out of their stasis tubes. Cloud was out of it for days due to heavy Mako poisoning, but lets go back to where He should have died from Sephiroth's sword? There was already small amount of Mako infused with him from the start but something made Cloud special before he'd even left his home town for Shinra in the hopes of becoming a soldier, what that was however is only speculation even from the creators point of view but never the less it was there... Genetics that allowed a stronger reaction to Mako and Jenova cells, cloud fought with himself even as a child and that was just with the slight Mako he absorbed from his home town Mako reactor, actually infusing Mako into his body with Jenova cells on top of that made his body and mind into something else altogether. Fighting Skills? lol I'd say they just came naturally to him after that, and how about Tifa who took a direct slash to the chest from Sephiroth's sword and survived? Now that's another story for another time lol

Let's assume Shinra did conduct basic training for their ground troops. I mean you come across guards that know a spell or two. He starts the game as Lv6, and Avalanche pay him next-to-nil for blowing up 2 reactors. They probably can't afford a Lv90 mercenary. Cloud's just lucky he's a rejuvinated corpse with Jenova cells or he'd be a lv1 grunt.

The reason he was able to stand-up to Sephiroth before he died? Pure luck when he was filled with rage, I think. He also caught Sephiroth off-guard.